This invention relates to a ferritic chromium alloyed steel formed from a melt having an as-cast fine equiaxed grain structure. More particularly, this invention relates to a ferritic chromium alloyed steel formed from a melt containing sufficient titanium and nitrogen but a controlled amount of aluminum for forming small titanium oxide inclusions to provide the necessary nuclei for forming the as-cast equiaxed grains. A hot processed sheet produced from the steel having this equiaxed cast grain structure is especially suitable for producing a cold reduced, recrystallization annealed sheet having excellent non-ridging characteristics and stretch formability, even without a hot band anneal or intermediate anneal.
It is desirable for a highly formable ferritic stainless steel, in addition to having a high plastic strain ratio, to minimize a phenomenon known as “ridging”, “roping” or “ribbing”. Unlike austenitic stainless steel, unsightly ridging may appear on the surfaces of a cold reduced, recrystallization annealed ferritic stainless steel sheet after being cold formed into a part. Ridging is characterized by the formation of ridges, grooves or corrugations which extend parallel to the rolling direction of the sheet. This defect not only is detrimental to the surface appearance of the sheet but also results in inferior stretch formability.
Ferritic chromium alloyed steels, especially sub-equilibrium ferritic chromium alloyed steels such as stainless Type 409 and 439, regardless of whether continuously cast into slab thicknesses of 50-200 mm or strip cast into thicknesses of 2-10 mm, typically have as-cast large columnar grains. These large columnar grains have a near cube-on-face crystallographic texture which leads to a very undesirable ridging characteristic in a final cold rolled, annealed sheet used in various fabricating applications. The surface appearance resulting from this ridging is highly objectionable in exposed formed parts such as caskets, automotive trim, exhaust tubes and end cones, stamped mufflers, oil filters, and the like. Ridging causes the sheet to have a rough, uneven surface appearance after forming and is attributed to a large non-uniform or “banded” grain structure present after cold rolling and annealing, resulting from the initial occurrence of the columnar grain structure in the as-cast steel.
To minimize the occurrence of ridging, additional expense is incurred by annealing a hot rolled sheet prior to cold reduction. This additional annealing step of hot rolled ferritic stainless steel also results in reduced formability caused by lower average strain ratios, i.e., Rm, which degrades deep drawability. A hot rolled sheet that is annealed before cold reduction must be cold reduced at least 70% to offset the loss of Rm caused by the hot band anneal before final annealing.
Over the years, there have been numerous attempts to obviate the above mentioned processing requirements and expense to eliminate ridging by modifying the alloy composition of ferritic stainless steel. It is known ridging in a ferritic stainless steel originates primarily during hot rolling. There have been attempts to minimize ridging by forming a fine equiaxed grain structure in a cast ingot by controlling the chemistry of the melt, e.g., one or more of the impurities of C, N, O, S, P, and by refining grain structure by using lower hot rolling temperatures, e.g., 950-1100° C. Chemistry control during refining has produced some improved ridging characteristics for ferritic stainless steels because of the formation of a second phase, i.e., austenite at elevated temperatures which becomes martensite at room temperature. However, formation of this second phase has been at the expense of tensile elongation and welding performance of the final products. Temperature control during hot rolling has resulted in operational difficulties as well since higher hot rolling power is required. Accordingly, hot roll sheet thicknesses must be greater. Hot rolling then must be followed by cold rolling in at least two stages with a second intermediate anneal between the two cold rollings.
U.S. Pat. No. 5,769,152 recognizes columnar grains are not desirable in continuously cast stainless steel. This patent suggests columnar grains can be prevented and equiaxed grains formed instead by casting molten steel using a low super heat temperature of 0-15° C. above the liquidus and magnetically stirring the molten steel in a casting mold.
Others have attempted to eliminate ridging by modifying an alloy composition of ferritic stainless steel by the addition of one or more stabilizing elements. U.S. Pat. No. 4,465,525 relates to a ferritic stainless steel having excellent formability and improved surface quality. This patent discloses that boron in amounts of 2-30 ppm and at least 0.005% aluminum can increase the elongation and the Rm as well as decrease the ridging characteristic. U.S. Pat. No. 4,515,644 relates to a deep drawing ferritic stainless steel having improved ridging quality. This patent discloses that an addition of aluminum, boron, titanium, niobium, zirconium and vanadium all can increase ferritic stainless steel elongation, increase the Rm and enhance the anti-ridging property. More specifically, this patent discloses a ferritic stainless steel having at least 0.01% Al that has improved anti-ridging characteristics. U.S. Pat. No. 5,662,864 relates to producing a ferritic stainless steel having good ridging characteristics when Ti, C+N and N/C are carefully controlled. This patent teaches ridging can be improved due to formation of carbonitrides by adding Ti in response to the C+N content in a melt. The steel melt contains ≦0.01% C, ≦1.0% Mn, ≦1.0% Si, 9-50% Cr, ≦0.07% Al, 0.006≦C+N≦0.025%, N/C≧2, (Ti−2S−3O)/(C+N)≦4 and Ti×N≦30×10−4. U.S. Pat. No. 5,505,797 relates to producing a ferritic stainless steel having reduced intra-face anisotropy and an excellent grain structure. This patent teaches good ridging characteristics are obtained when the steel melt preferably contains 0.0010-0.080% C, 0.10-1.50% Mn, 0.10-0.80% Si, 14-19% Cr and two or more of 0.010-0.20% Al, 0.050-0.30% Nb, 0.050-0.30% Ti and 0.050-0.30% Zr. The steel is cast into a slab and hot rolled to a sheet having thickness of 4 mm, hot strip annealed, pickled, cold rolled and finish annealed. The slab was heated to 1200° C. and subjected to at least one rough hot rolling pass at a temperature between 970-1150° C. The friction between the hot mill rolls and the hot rolled steel was 0.3 or less, the rolling reduction ratio was between 40-75% and the hot rolling finishing temperature was 600-950° C. The hot rolled steel was annealed at a temperature of 850° C. for 4 hours, was cold reduced 82.5% and finish annealed at a temperature of 860° C. for 60 seconds.
It is known when the solubility product of titanium compounds exceeds the saturation level at the liquidus temperature, i.e., hyper-equilibrium, for titanium stabilized stainless steels, the titanium compounds are stable and TiN will precipitate before freezing of the metal. Steel sheet produced from these hyper-equilibrium slabs exhibit improved ridging characteristics and formability. Upon freezing, however, the TiN coalesced into large clusters and floated to the surface of the cast slab. These non-metallic TiN clusters formed unacceptable open surface defects known as a Ti-streaks during hot rolling. These large non-metallic clusters must be removed from the slab by costly surface conditioning such as grinding prior to hot processing of the slab. U.S. Pat. No. 4,964,926 relates to weldable dual stabilized ferritic stainless steel having improved surface quality by eliminating the formation and precipitation of non-metallic titanium oxides and titanium nitrides during casting by forming a sub-equilibrium titanium stabilized ferritic stainless steel. This patent discloses it was known that roping characteristics could be improved by adding niobium alone or niobium and copper to a ferritic stainless steel. However, the addition of niobium alone caused weld cracking. U.S. Pat. No. 4,964,926 discloses replacing a portion of a titanium stabilizer with a niobium stabilizer to form a dual stabilized ferritic stainless steel. An addition of at least 0.05% titanium to a niobium stabilized steel eliminates weld cracking.
Pending U.S. patent application Ser. No. 08/994,382, filed Dec. 19, 1997, entitled “Non-Ridging Ferritic Chromium Alloyed Steel”, now U.S. Pat. No. 5,868,875 incorporated herein by reference, relates to a titanium deoxidized, ferritic chromium alloyed steel formed from a melt having an as-cast fine equiaxed grain structure. This application discloses a ferritic chromium alloyed steel formed from a melt deoxidized with titanium and containing no greater than 0.01 wt. % aluminum. A hot processed sheet produced from steel having this as-cast equiaxed grain structure is especially suitable for a cold reduced, recrystallization annealed sheet having excellent formability, stretching and non-ridging characteristics without an extra processing step, such as hot band anneal or an intermediate anneal.
The minimization of ridging by prior artisans has sacrificed cost and formability by annealing hot rolled ferritic stainless steel prior to cold reduction. This additional annealing step reduces formability by lowering the average Rm. Also, this pre-annealed hot rolled steel must be cold reduced at least 70% to obtain an Rm after final annealing similar to the Rm for a hot rolled steel that otherwise is not annealed before cold reduction. This greater percentage cold reduction generally requires an intermediate annealing step as well. As evidenced by the seemingly endless struggle of others, there remains a long felt need for an annealed ferritic chromium alloyed steel essentially free of ridging and having excellent deep formability characteristics such as a high Rm, a high tensile elongation and a uniformly annealed grain structure. There remains a further need for an excellent deep formability ferritic stainless steel having good ridging characteristics that does not require a hot processed sheet to be annealed prior to cold reduction. There remains a further need for an excellent deep formability sub-equilibrium, ferritic stainless steel having good ridging characteristics formed from a hot processed sheet that does not have surface defects, i.e., titanium nitride scale and titanium oxide streaks, without requiring surface conditioning of the surfaces of a continuously cast slab prior to hot processing of the slab.